Europe’s greenest and cleanest cities

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Traffic jams, smog, litter-strewn streets. City life can be hard at the best of times, and with the United Nations predicting that the number of city dwellers will reach 6.25 billion by 2050, the challenge to develop sustainable, livable and green cities is perhaps greater than ever before.

In 2011, the UN released research which reported that, 'the world's cities [are] responsible for up to 70 percent of harmful greenhouse gases while occupying just two percent of its land'.

Yet some cities are attempting to do things right. Since 2010, the European Commission has awarded the European Green Capital Award to cities showing willingness to use innovation and technology to create green, healthy and energy efficient cities.

Here, CNBC.com takes a look at the award's winners, and finds out what makes them so special.